feat: preserve speed history and improve account setup

tests/download-speed-state.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import { updateDownloadSpeedHistory } from "../src/renderer/download-speed-state";
+
+describe("download speed history", () => {
+  it("keeps the smoothed display and history across successive updates", () => {
+    const first = updateDownloadSpeedHistory({ history: [], display: 0 }, 100, 10);
+    const second = updateDownloadSpeedHistory(first, 100, 10);
+
+    expect(second.display).toBeGreaterThan(first.display);
+    expect(second.history).toHaveLength(2);
+    expect(second.history[0]).toBe(first.display);
+  });
+
+  it("keeps only the configured number of samples", () => {
+    let state = { history: [] as number[], display: 0 };
+    for (let index = 0; index < 5; index += 1) {
+      state = updateDownloadSpeedHistory(state, index + 1, 3);
+    }
+
+    expect(state.history).toHaveLength(3);
+  });
+});